The present invention relates to a method and unit for controlling the supercharge pressure of a turbodiesel engine with a variable-geometry turbine.
As is known, turbodiesel engines equipped with a variable-geometry turbine provide for more power as compared with traditional types. Such turbines, in fact, permit more rapid response and increased torque at low engine speed, and greater power at high engine speed.
For a clearer understanding of the problems dealt with by the present invention, a turbosupercharger featuring a variable-geometry turbine will now be described with reference to FIG. 1, which shows a turbosupercharger of the type in question, in particular a Garrett VNT25 TD2502.
Number 1 in FIG. 1 indicates a turbosupercharger comprising a spiral-shaped inlet conduit 2 for the exhaust gas produced by combustion in an engine (not shown); a turbine 3 activated by the exhaust gas; a first outlet conduit (not shown) for expelling the exhaust gas after activating the turbine; a second inlet conduit (not shown) for the intake of air; an air compressor 4 activated by turbine 3; and a second outlet conduit 5 for supplying the compressed air to the engine.
Turbine 3 comprises a bladed rotor 6, and a variable-geometry blade system 7 comprising a number of blades 8 located between first inlet conduit 2 and bladed rotor 6.
Blades 8 are adjustable, and are activated by a control assembly 9 comprising a diaphragm actuating member 10 and an actuating assembly (not shown in FIG. 1) for activating actuating member 10 and described in detail later on with reference to FIG. 2.
In actual use, blades 8 vary the section of the exhaust gas passage as a function of the output pressure of compressor 4, regulate the speed of the exhaust gas, and continuously control the supercharge pressure.
The position of blades 8 depends on the operating conditions of the engine. More specifically, at low engine speed and under low load conditions, blades 8 are set to the fully closed position to reduce the section of the gas passage and increase the speed of the gas in turbine 3.
Conversely, at high engine speed, the section of the gas passage through blades 8 is increased, so that the gas flows over the rotor at a slower surface speed, thus reducing the rotation speed of turbine 3, which eventually settles at the right speed for ensuring correct operation of the engine.
As shown in FIG. 2, the actuating assembly--indicated by 11--for activating actuating member 10 comprises a vacuum pump 12 activated by the engine--indicated by 13--to which turbosupercharger 1 is fitted; and a known vacuum-modulating three-way solenoid valve 14 connected to vacuum pump 12 by an input line 15.
More specifically, vacuum pump 12 is driven by the camshaft 13a or the drive shaft (not shown) of engine 13.
Solenoid valve 14 is connected to actuating member 10 by an output line 16, and is controlled by a control unit 17.
Control unit 17 comprises a pressure sensor 18 and an electronic central control unit 19.
Pressure sensor 18 is located along second outlet conduit 5 of turbosupercharger 1, and generates a pressure signal PSmis proportional to the supercharge pressure in second outlet conduit 5 of supercharger 1.
Electronic central control unit 19 receives pressure signal PSmis and a reference signal PSrif proportional to a desired supercharge pressure value and mapped in a memory (not shown), and generates a control signal DutyV related to the optimum opening angle of blades 8 in FIG. 1.
Control signal DutyV is supplied to actuating assembly 11 of actuating member 10, and more specifically to solenoid valve 14.
In actual use, vacuum pump 12 generates a vacuum, which is supplied to and modulated by solenoid valve 14 according to the value of control signal DutyV, so as to control the diaphragm of actuator 10 in known manner (not shown) and so adjust the position of blades 8 in FIG. 1.
In traditional control systems, control signal DutyV of solenoid valve 14 is generated by a known proportional-integral control--not described in detail--which performs a transfer function having, for the proportional and integral part respectively, two multiplication coefficients Kp and Ki, which assume a fixed value regardless of the operating point of the engine.
Proportional-integral control, however, is poorly suited to controlling the power output of a turbodiesel engine with a variable-geometry turbine during acceleration, in that it is slow and acts on blades 8 (FIG. 1) with a delay which is incompatible with the sudden increase in power of the engine at the acceleration stage.
The delay referred to is manifested by the supercharge pressure overshooting the desired value, and subsequently settling at the desired value accompanied by oscillations with a small damping coefficient.